Possible Clues:

• Illinois city symbolizing middle America
• Test marketer's city
• Midwest city founded as Fort Clark
• Noted spot in test-marketing
• Home of Caterpillar, Inc
• City by the Illinois River
• Illinois city that symbolizes Middle America
• Where Betty Friedan and Richard Pryor were born
• Midwest city that shares its name with a suburb of Phoenix
• 'Will it play in __?': vaudeville phrase